The present invention relates to a circuit arrangement, especially for supplying power to an electromagnetic holding brake with a clocked supply voltage.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
One of the items disposed on a powerful electric motor, as is used for example as a variable-speed drive of an industrial machine, is an electromechanical holding brake. The brake serves to brake or to stop the motor shaft quickly and safely in a safety-critical situation (system error, power outage, . . . ) and thus put it into a state which is safe for people and for the system. The brake is also frequently used as a holding brake in order, when a motor is switched on, to prevent to motor shaft starting to turn by itself as a result of external influences.
Such an electromagnetic holding brake is often embodied in the form of what is referred to as a spring-pressure brake. This generally comprises a permanent magnet, an inductance, a spring and friction surfaces (brake surfaces). The inductance and the permanent magnet are attached to a bearing end shield, the friction surface with the spring is attached to the motor shaft. In the no-load state the friction surfaces are pressed together via the field of the permanent magnet.
A spring-pressure brake of this type is pretensioned by a spring force into a braking position. In its idle state it is thus in the brake position and can be released from this brake position (“released”) by the application of a voltage in order to free the motor shaft.
A specific voltage is required to release the spring-operated brake. If the voltage applied is too great the field of the permanent magnet is overcompensated by the inductance and the brake drops back into the braking position. The rated braking voltage has very narrow tolerances, for example 24V (DC)±10%.
In principle however, after a release process in which the spring-pressure brake has been released, the braking voltage present is reduced from the rated voltage to what is referred to as the holding voltage (for example 16V (DC)). After a release process the air gap produced and also the spring resetting force are sufficient to hold the brake in a released position with the holding voltage or the holding current respectively.
Electromagnetic holding brakes for electrical drives are known from the prior art in which a buck converter is provided to generate the braking voltage which is controlled by a braking voltage setpoint value pulse width modulated with a pulse width modulator. In such an arrangement it is possible, by variation of the duty cycle, to control the braking voltage. In particular it is possible to reduce the voltage at the holding value to below the rated voltage in that the duty cycle of the pulse width modulation is reduced. This enables the power loss during the released mode to be explicitly reduced.
In order to ensure secure brake management a suppressor diode is typically connected in a drive in parallel to the inductance of the holding brake. With a rapid application of the holding brake or with a rapid de-magnetization respectively the windings of the holding brake inductance aim to maintain the current flowing through them (free-wheeling current) so that in some circumstances very high overvoltage peaks occur. The suppressor diode suppresses the overvoltage pulses arising through the inductance (for example to 39V), in that the surplus energy is consumed by it or conducted by it. The suppressor diode thus protects the holding brake as well as the control electronics against damage.
If the holding brake is controlled with a pulse width-modulated voltage signal the problem arises here of the suppressor diode having free-wheeling current permanently applied to it, wherein the power loss arising in such cases cannot be removed.
It would be desirable and advantageous to provide an improved circuit arrangement for an electromagnetic holding brake and to enable control of the holding brake with a clocked supply voltage and to enable rapid application of the holding brake.